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The development of novel conversion technologies for recovering waste heat is one of the technical goals of the European Commission, considering the large amount of thermal energy discharged by industrial processes in Europe. The recently launched EU-H2020 project CO2OLHEAT aims at contributing to industrial waste heat recovery by developing a novel sCO2 power system of 2 MW power capacity. Considering the wide range of conditions featuring waste heat recovery applications (in terms of both flue gas temperature and flow rate), as well as the flexibility of operation required by such installations, a simple recuperative cycle was selected for the CO2OLHEAT plant. The system, however, features a relatively non-conventional multi-shaft configuration, composed of a radial-inflow turboexpander, which drives the near-critical compressor, and by an axial-flow turbine to generate the 2 MW output power. The present study focuses on the design of the radial turbines of the CO2OLHEAT turbo-expander. Due to the low volumetric flow rate, the turbine features small size and, therefore, significant aerodynamic challenges have to be considered in the design process. A preliminary design of the machine was performed by applying a mean-line approach, then a conceptual design of the bladings and of the meridional channels was performed by resorting to industrial in-house database and criteria. Once the turbine design was finalized, the aerodynamics and performance of the machine were evaluated by applying two different computational fluid dynamic simulation tools, which exhibit remarkable agreement. One CFD tool was then used to investigate and quantify the impact of wheel tip clearance and surface roughness, as well as to quantify the deviation with respect to mean-line predictions. Relevant quantitative data are extracted as well as aerodynamic indications are inferred, with the aim of establishing a reference for the future design of compact sCO2 radial turbines., Conference Proceedings of the European sCO2 Conference5th European sCO2 Conference for Energy Systems: March 14-16, 2023, Prague, Czech Republic, p. 272

The development of novel technical solutions for the effective recovery of waste heat is crucial for making accessible the enormous amount of thermal energy released by industrial processes, thus supporting the EU energy strategy. To this end, the EU-H2020 project CO2OLHEAT aims at developing, and demonstrating in a real industrial environment, a novel sCO2 power unit of 2-MW capacity recovering energy from flue gases at 400 °C. The thermo-economic optimization of the system and the complexity of its implementation led to select a simple recuperative cycle for the CO2OLHEAT unit, which features a relatively unconventional multi-shaft configuration where the sCO2 compressor is driven by a dedicated radial expander, while the electrical power is generated via a separated axial turbine. The present study focused on the design and computational assessment of the compressor for the CO2OLHEAT system. The thermodynamic optimization of the cycle led to an overall pressure ratio slightly above 2.5, delivered with a two-stage centrifugal compressor. As typically found in sCO2 power systems, the thermodynamic state of the fluid at the machine intake (P = 85 bar; T = 32°C) is close to the critical point and to the saturation curve; therefore, the first stage of the machine demands a dedicated aero-thermodynamic design, which can account for the effects of non-ideal thermodynamics and of the potential onset of two-phase flows. The paper discusses the conceptual aero-mechanical design of the compressor and then focuses on its performance assessment over the full operating range via Computational Fluid Dynamics. Two alternative flow models are considered, the first one based on the experimentally-validated barotropic fluid representation, while the second one featuring a complete thermodynamic model which assumes homogeneous equilibrium between the phases. The approaches provide similar outcomes, showing that the compressor fulfills the system requirement and guarantees large rangeability., Conference Proceedings of the European sCO2 Conference5th European sCO2 Conference for Energy Systems: March 14-16, 2023, Prague, Czech Republic, p. 301

The present work investigates the part-load performance of a MW-scale sCO2 power plant designed as heat recovery unit for an existing cement plant located in Czech Republic, in the framework of the H2020 funded project CO2OLHEAT. The study firstly presents the selected power plant configuration and then focuses on the evaluation of its part load operation due to variation of flue gas mass flow rate and temperature. The range of flue gas conditions at the outlet of the upstream process is retrieved from a preliminary statistical analysis of historical trends obtained through the cement plant monitoring. The numerical model developed for this study aims at providing realistic results thanks to the adoption of turbomachinery performance maps provided by the project partners. Moreover, heat exchangers have been modelled through a discretized approach which has been validated against manufacturer data, while piping inventory and pressure losses have been assessed through a preliminary sizing that considers the actual distances to be covered in the cement plant. Performance decay is estimated for the whole range of flue gas conditions, reporting the most significant power cycle parameters, and identifying the main causes of efficiency loss. The part-load analysis is carried out considering a constant CO2 inventory, in order to reduce the system complexity and capital cost and simplify plant operation. Results show that the operation entails minor variation of the compressors operative points in the whole range of operating conditions of the cement plant, avoiding the risk of anti-surge bypass activation. Moreover, the plant is able to work at nearly constant thermodynamic cycle efficiency (20.5%-23.0%) for most of the year and benefits from part-load operation in terms of overall performance. These predictions will be used, in next steps of the project, to guide the definition of power plant control during transients related to changes of upstream process conditions or specific needs of power output control, Conference Proceedings of the European sCO2 Conference5th European sCO2 Conference for Energy Systems: March 14-16, 2023, Prague, Czech Republic, p. 179